Detectors responsive to electromagnetic radiation generally include an array of pixels, each pixel being operable to generate or pass a current in response to electromagnetic energy incident upon the pixel. Typically, the generated or passed current is proportional to the incident energy. The pixels may be prone to some fluctuation in their response behavior over the life of the detector, and there may also be variations from pixel to pixel. An unknown change in performance of the detector could lead to erroneous data regarding measurements of an electromagnetic energy source. Accordingly, although calibration in a laboratory environment is typically performed before deployment, regular re-calibration after deployment may be desired to accommodate for changes in pixel responses over time and ensure uniformity in measurements. Further, for multi-band or multi-spectral detector systems, it can be necessary to provide calibration at different wavelengths (e.g., in different spectral bands), which may require the use of two or more calibration sources. In particular, calibration of multi-band infrared imaging systems may use two or more “black body” calibration sources, each thermally controlled to particular temperature such that it emits a specified and well-controlled spectrum.
Conventional calibrators require multiple mechanisms to configure an imaging system into a calibration mode, in which electromagnetic radiation from a calibration source, rather than from a viewed scene, is directed to the detector, and to select between two or more calibration sources. For example, some calibrators use two mirrors, namely a “switch” mirror to intercept the light path to the detector, and a second “select” mirror to direct the light path to a particular calibration source. Another conventional calibrator design uses a single mirror, but places the calibration sources, each controlled to a particular temperature, on a rotating cradle, such that selection of one of the calibration sources is achieved by rotating the cradle. This arrangement presents difficult thermal challenges. U.S. Pat. No. 9,024,253 titled “CALIBRATION SYSTEM FOR DETECTOR” discloses another example of a multi-band calibrator that uses a single, one-axis, multi-position calibration mirror that is moved into different positions to select between one of three calibration sources.